1. Field of the Invention
This invention relates to continuous casting of a metal ribbon, excelling in ease of fabrication and consisting of a unidirectional solidification structure and characterized by a substantially greater length than diameter or thickness, e.g., in the form of a metal strip or a metal wire.
More particularly, this invention relates to a process for the continuous casting of a metal strip or ribbon by feeding a molten metal onto the structure of a solidification base or support moving generally continuously in one direction at a locus along its path, which process is characterized by preheating the solidification base upstream of the locus for receiving the molten metal to a temperature above the fusion point of the metal, thereby preventing the received molten metal from forming crystal cores or nuclei upon contact with the support surface, and cooling the metal to solid condition on said support surface as the same moves away from the feeding locus conferring upon the resultant solid metal ribbon a unidirectional solidification structure excelling in ease of fabrication.
2. Description of the Prior Art
Recently, the rapid advance of the electronic industry has produced a persistent pressure for reduction in size and improvement of accuracy in machines and implements used therein. Along with this pressure, metal materials used therefor have been expected to fulfill their function satisfactorily with increasingly small thickness or width and increasingly exacting quality. To be specific, there has developed a need for fine lines and thin sheets and foils made of a metallic material possessing a unidirection solidification structure devoide of gross porosity, bubbles, or crystal grain boundaries which are liable to collect deposits of impurities.
It is generally known that when a metal ribbon is subjected to cold working, such as cold rolling or cold drawing, it undergoes work hardening and eventually fractures along primary crystal grain boundaries which were formed during the course of solidification of the metal ribbon. It is, therefore, highly desirable that metal ribbon serving as the starting material for an extremely fine line or an extremely thin sheet or foil should possess a texture free of primary crystal grain boundaries which are liable to initiate the generation of cracks by such working as described above.
This invention aims to provide a process capable of continuously producing a metal ribbon possessing a unidirectional solidification structure readily adapted for working as by rolling or drawing and containing no internal defects such as gross porosity and bubbles, by an extremely simple operation of feeding a molten metal via a nozzle onto the surface of a solidification base or support being continuously moved in one direction.
Heretofore, a metal strip has been widely produced for the manufacture of amorphous ribbon by continuously feeding molten metal from a nozzle to the surface of a cooled solidification base or support in the shape of a cylinder or drum being rotated in one direction thereby allowing the molten metal to be suddenly cooled and solidified. The same method is also now used generally for the manufacture of thin metal strip in addition to the aforementioned amorphous metal ribbon. The metal strip obtained in this way, however, has a polycrystalline form because the molten metal forms crystal cores or nuclei on contact with the surface of the cooled solidification support. Moreover, the crystals so produced are liable to grow in parallel directions substantially perpendicularly to the surface of the solidification support. Since the metal strip in polycrystalline form is liable to sustain cracks along crystal grain boundaries while being worked, manufacture of a very thin foil and fabrication into an extremely slender line has been attained only with great difficulty. Particularly, a strip of an alloy having a wide solidification temperature range is very likely to sustain cracks along crystal grain boundaries. It has been found difficult, therefore, to attain smooth separation of the strip of this alloy from the curved solidification support without creating cracks in the strip.